1. Field of the Invention
The present invention is related, generally, to ball joint assemblies and, more particularly, to ball joint assemblies of the type used in automotive steering and suspension systems.
2. Related Art
Vehicle steering and suspension systems typically include one or more ball joints, such as tie rod end ball-type joints for operable attachment of a tie rod end to a steering knuckle. One typical type of ball joint is a compression loaded ball joint which includes a ball stud with a generally spherical ball portion which is captured by one or more bearings that are contained within a housing.
During use, the spherical ball portion of the ball stud is subjected to both varying axial loads and varying radial loads. The axial forces are applied in a downward direction and compress the lower bearing while the radial forces cycle in various directions. Additionally, the ball stud may be subjected to rotational forces about its axis which cause the ball stud to rotate relative to the bearings and the housing. The application of these rotational forces are applied while the ball stud is under axial and/or radial loads results in wear between the spherical ball portion of the ball stud and the bearings (which are usually of plastic or metal) that capture the ball portion.
Typically, the contact angle and the clearance between the outer surface of the spherical ball portion of the ball stud and the contact surfaces of the bearings affects the resulting rotational torque required to rotate the ball stud. It is preferred to provide the ball joint with minimal clearance to limit movement of the ball portion relative to the bearing in both the axial and radial directions while also minimizing the rotational torque. In order to minimize the clearance between the ball portion of the ball stud and the bearings, some ball joint manufacturers provide the curved contact surfaces of the bearings with a very similar diameter to the diameter of the ball portion. However, wear in the ball portion and in the bearings may cause the contact angles and the clearances to change over time, thereby increasing the rotational torque values.
Another approach that ball joint manufacturers have taken with the goal of limiting the change in rotational torque values as the ball portion and the bearings wear from use is to provide the curved contact surfaces of the bearings with a greater diameter than the diameter of the ball portion. This approach has the effect of minimizing the contact areas and angles between the ball portion and the contact surfaces of the bearings, thereby limiting the increase in rotational torque values as the bearings and ball portion wear. However, this approach also results in increased clearance between the ball portion and the bearings, which may reduce the expected life of the ball joint.